It is known that magnetic separators are used in all those applications where it is necessary to attract and separate ferromagnetic materials of any shape and size from mixed material. The attractive capacity of the separator depends both on the magnetic field that it can generate (strength and gradient), and on the intrinsic induction of the object to be separated as it results from its shape factor (e.g. the sphere has the worst shape factor) and from its degree of permeability.
Attractive circuits (i.e. permanent magnets) made of ceramic materials such as barium ferrite, and even better strontium ferrite, are known since more than forty years. These magnets have a medium intrinsic and residual magnetic energy, and are capable of attracting within a certain distance ferromagnetic materials with high shape factor and/or medium-high permeability.
Other attractive circuits made of sintered materials with high intrinsic residual magnetic energy, known as rare earth elements (samarium-cobalt, iron-boron-neodymium), have been in use more recently, in the last 15-20 years. These magnets can attract within a relatively short distance, yet with great effectiveness, even materials with low shape factor and/or medium-low and very low permeability. Their effectiveness is however concentrated within few tens of millimeters.